Tutorial: Turned Lapped Seam by ohbabylee, It's very strong, as the seam is sewn three times. The raw seam allowances are hidden inside so it can be used on all kinds of fabrics. The seam gives a neat finish which is identical on both sides so it is perfect for reversible garments. You can also layer two fabrics together to feature a different fabric on each side. You do end up with four layers of fabric in the seam, so it is not ideal for very thick fabrics.